Radio frequency identification (RFID) systems operating in the high-frequency range, typically at 13.56 Megahertz (MHz), are radiation limited by governmental regulations, such as the Federal Communications Commission (FCC) rules governing the industrial, scientific, and medical (ISM) operating bands commonly used for these unlicensed systems, in particular 47CFR15.225. These RFID systems are commonly known as vicinity readers because they are capable of reading credit card sized RFID tags to a distance of 60 centimeters (about two feet).
As is known in the art, antenna systems have near-field and far-field radiation regions. The near field is a region near an antenna where the angular field distribution depends upon the distance from the antenna. The near field is generally within a small number of wavelengths from the antenna and is characterized by a high concentration of energy and energy storage in non-radiating fields. In contrast, the far field is the region outside the near field, where the angular distributions of the fields are essentially independent of the distance from the antenna. Generally, the far-field region is established at a distance of greater than D2/λ from the antenna, where D is an overall dimension of the antenna that is large compared to wavelength λ. The far-field region is where radiation from the antenna is said to occur.
RFID systems use near fields for communications between the RFID tag and the RFID interrogator. Also, the energy stored in the near fields provides the power to drive a microchip imbedded in a passive RFID transponder tag. Many conventional RFID systems use loop-type radiators for interrogator antennas, for example, an antenna consisting of a figure-eight shaped conductor.
Conventional RFID systems are being increasingly used to enhance supply chain activities, security, and a myriad of other applications and industries. However, conventional RFID systems often have limited operating ranges, which limits their usefulness. Attempts to increase RFID system range, however, often result in the need for increasing input power, which violates FCC radiation limitations, generally because of proportional increases in far-field radiation.
It would, therefore, be useful to provide a RFID system that can increase near fields while simultaneously reducing far-field radiation. Such a RFID system would have an increased operating range while abiding by applicable governmental RF radiation regulations.